The present invention relates to a solid electrolyte high-temperature fuel cell arrangement consisting of several fuel cells which are arranged directly adjacent one another and connected in series and which are formed by adjacently disposed planar connecting plates with solid electrolyte elements arranged therebetween and wherein the connecting plates of two adjacent cells provide for electrical connections between the cathode of one cell and the anode of the other cell and include gas channels and gas admission means as well as air channels and air admission means.
In a fuel cell, fuel gas is converted electrochemically with oxygen whereby electrical energy is directly generated. The reaction partners are supplied in different channels, that are the fuel and air channels, separated by the ceramic solid electrolyte element which is provided at both sided with electrodes. During operation electrons are emitted at the fuel side electrode of the solid electrolyte whereas electrons are absorbed at the oxygen side electrode whereby a potential difference is generated between the two electrodes. The solid electrolyte separates the reactants, it transfers the charge in the form of ions and, at the same time, presents an electron short circuit between the two electrodes of the solid electrolyte. For this purpose, the solid electrolyte needs to have a low conductivity for electrons but, at the same time, a high conductivity for ions.
In contrast to low temperature fuel cells, solid electrolyte high temperature fuel cells are suitable for the conversion of not only hydrogen but also for the conversion of hydrocarbons such as natural gas or propane which can be stored in liquid form. With solid electrolyte, high temperature fuel cells high power densities on the order of several 100 mW/cm.sup.2 can be achieved. A single high temperature fuel cell generates an idle voltage of about 0.7. For higher voltages a serial arrangement of several single cells is required.
A solid electrolyte high temperature fuel cell arrangement of the type referred to initially is known for example from DE-OS-39 35 722. Herein four stacks of plates are mounted in a horizontal arrangement in a frame of insulating material wherein gas and air are supplied to the fuel cells and, respectively, removed therefrom through the frame.
Further solid electrolyte fuel cell arrangements are known from U.S. Pat. No. 4,476,198, DE 40 09 138 and DE 34 37 354.
DE 40 09 138 discloses a solid electrolyte high temperature fuel cell module comprising several serially arranged planar solid electrolyte high temperature fuel cells which are disposed directly on one another. A bi-polar plate is provided which electrically interconnects the cathode of one cell and the anode of an adjacent cell. Also, gas and air channels are formed by the bipolar plate. Solid electrolyte plates have cathode material at one side and anode material at the other. In order to prevent thermal tensions, the bipolar plate consists of a metallic conductive alloy whose thermal expansion coefficient corresponds to that of the solid electrolyte element.
The European patent application 0 473 540 discloses a fuel cell arrangement wherein the fuel cell elements are disc-shaped. By a particular arrangement of the gas channels a heat exchange between the supply air and the exhaust air and gas is achieved. A subsequent combustion of the gas and the air can be provided for immediately following the discharge of the gas and the air from the fuel cells.
However, because of the circular disc-like arrangement, the gas flow is radial whereby the gas flow density increases or decreases rapidly with the radius. As a result, the heat exchanger and also, the gas reactions within the cell cannot be designed for a particular gas speed.
Furthermore, with a combustion of the gas leaving the cell stack the cells are disposed practically in the center of the flame whereby the cells are heated to unnecessarily high temperatures. Since the flame extends radially outwardly and the combustion gas rises by thermal effects, the upper part of the cell stack is heated to a much greater degree than the lower part. Such uneven heating results in high thermal tensions in the cell stack.
Another disadvantage results from the unavoidable gas/air parallel flow at the electrolyte plate (PEN-plate) which is provided at one side with the positive cathode and at the other side with the negative anode, also a gas/air cross-flow arrangement is impossible to establish. A gas admission in the center, but particularly an air admission at the large outer circumference of the cell stack is very difficult.
Another big problem with the fuel cell technology is the sealing of the joints. In the fuel cell of the last mentioned patent application for example the sealing problems are aggravated because the lower joints are more heavily loaded than the upper joints and because bending stresses may occur at the clamping locations.
It is the object of the present invention to provide a solid electrolyte high temperature fuel cell of the type referred to above without however the disadvantages mentioned above.